IN SITU, ON-SITE AND LABORATORY MEASUREMENTS OF SOIL AIR PERMEABILITY: BOUNDARY CONDITIONS AND MEASUREMENT SCALE

Abstract

The measurement of soil air permeability is a fast and easy method that can be used in different ways to characterize the soil. An air permeameter was constructed in order to measure air permeability (k_a) in situ, on-site (exhumed soil samples), and in the laboratory on a wide range of Danish agricultural soils. Two different sizes of sample rings were used (100 cm³ and 3140 cm³). The device was initially tested in the laboratory on repacked soil samples to evaluate dependency of k_a on sample size. The results showed consistent values of k_a for both sample sizes, indicating only little scale effect. In the field, air permeability was measured in situ and on-site using large sample rings. Air permeability in situ was determined by using a "shape factor" taking into account boundary conditions at the lower end of the ring while assuming isotropic soil conditions. An expression for the shape factor developed by Liang et al. (1995) was used. The results from the two measurement methods compared well, indicating reliable air permeability values using the expression of Liang et al. for the soils studied. Air permeability in structured soil measured using exhumed samples of different size showed that small samples generally yielded lower values and higher variability in k_a than large samples, in accordance with the concept of a representative elementary volume.